Unraveling mechanisms of the uncoordinated nucleophiles: theoretical elucidations of the cleavage of bis(p-nitrophenyl) phosphate mediated by zinc-complexes with apical nucleophiles

Abstract

A theoretical approach was used to investigate the hydrolytic cleavage mechanisms of the bis(p-nitrophenyl) phosphate (BNPP⁻) catalyzed by Zn(II)-complexes featuring uncoordinated nucleophiles. Ligand-based and alternative solvent-based nucleophilic attack reaction models are proposed. The pK_a values of the Zn(II)-bound water molecules or ligands in the [Zn(LⁿH)(η-H₂O)(H₂O)]²⁺ (n = 1, 2 and 3) complexes, as well as the dimerization tendency of the mononuclear Zn(II)-complexes, were found to significantly influence the reaction mechanisms. The Zn(II)-L³ complexes were found to be more favorable for the hydrolytic cleavage of the BNPP⁻ via a ligand-based nucleophilic attack pathway. This was due to the lower pK_a value for the deprotonation of the oxime ligand, the hard dimerization of the mononuclear Zn(II)-L³ species, and the presence of an uncoordinated nucleophile. The origins of the uncoordinated reactions were systematically elucidated. The theoretical results reported here are in good agreement with experimental observations and more importantly, help to elucidate the factors that influence intermolecular nucleophilic attack reactions with coordinated/uncoordinated nucleophiles.